feat($resource): Make stripping of trailing slashes configurable.

First, this now uses a flat object configuration, similar to
`$httpBackend`.  This should make configuring this provider much more
familiar.

This adds a fourth optional argument to the `$resource()` constructor,
supporting overriding global `$resourceProvider` configuration.

Now, both of these ways of configuring this is supported:

    app.config(function($resourceProvider) {
      $resourceProvider.defaults.stripTrailingSlashes = false;
    });

or per instance:

    var CreditCard = $resource('/some/:url/', ..., ..., {
        stripTrailingSlashes: false
    });